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Enqueues are similar to locks in that they are internal mechanisms that control access to resources. High enqueue waits indicate that a large number of sessions are waiting for locks held by other sessions. You can query the dynamic performance view V$ENQUEUE_STAT to find out which of the enqueues have the most wait times reported. You can do this by using the cum_wait_time (shows the cumulative time spent waiting for the enqueue) column of the view. Note that the use of locally managed tablespaces eliminates several types of enqueues such as space transactions (ST) enqueues. In a system with a massive concurrent user base, most common enqueues are due to infrequent commits (or rollbacks) by transactions that force other transactions to wait for the locks held by the early transactions. In addition, there may be a problem with too few interested transactions list (ITL) slots, which also show up as transaction (TX) enqueues. Locally managed tablespaces let you avoid the most common types of space-related enqueues.
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Latches are internal serialization mechanisms used to protect shared data structures in Oracle s SGA. You can consider a latch as a type of lock that s held for an extremely short time period. Oracle has several types of latches, with each type guarding access to a specific set of data. The latch free wait event is incremented when a process can t get a latch on the first attempt. If a required Oracle latch isn t available, the process requesting it keeps spinning and retrying to gain the access. This spinning increases both the wait time and the CPU usage in the system. Oracle uses about 500 latches, but two of the important latches that show up in wait statistics are the shared pool latch (and the library cache latches) and the cache buffers LRU chain. It s normal to see a high number of latch free events in an instance. You should worry about this wait event only if the total time consumed by this event is high. High latch waits will show up in your AWR reports, or you can use the query shown in Listing 20-17 to find out your latch hit ratio. Listing 20-17. Determining the Latch Hit Ratio SQL> SELECT a.name "Latch Name", a.gets "Gets (Wait)", a.misses "Misses (Wait)", (1 - (misses / gets)) * 100 "Latch Hit Ratio %" FROM V$LATCH a WHERE a.gets != 0 UNION SELECT a.name "Latch Name", a.gets "Gets (Wait)", a.misses "Misses (Wait)", 100 "Latch Hit Ratio"
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CHAPTER 20 PERFOR MAN CE TUNING: TUNING THE INSTA NCE
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FROM V$LATCH a WHERE a.gets = 0 ORDER BY 1; SQL> If the ratio isn t close to 1, it s time to think about tuning the latch contention in your instance. There s only one shared pool latch for the database, and it protects the allocation of memory in the library cache. The library cache latch regulates access to the objects present in the library cache. Any SQL statement, PL/SQL code, procedure, function, or package needs to acquire this latch before execution. If the shared pool and library cache latches are high, more often than not that s because the parse rates in the database are high. The high parse rates are due to the following factors: An undersized shared pool Failure to use bind variables Using dissimilar SQL statements and failing to reuse statements Users frequently logging off and logging back into the application Failure to keep cursors open after each execution Using a shared pool size that s too large The cache buffers LRU chain latch free wait is caused by high buffer cache throughput, either due to full table scans or the use of unselective indexes, which lead to large index range scans. Unselective indexes can also lead to yet another type of latch free wait: the cache buffer chain latch free wait. These wait events are often due to the presence of hot blocks, so you need to investigate why that might be happening. If you see a high value for row cache object latch waits, it indicates contention for the dictionary cache, and you need to increase the shared pool memory allocation. In most instances, latch waits tend to show up as a wait event, and DBAs sometimes are alarmed by their very presence in the wait event list. As with the other Oracle wait events, ask yourself this question: Are these latch waits a significant proportion of my total wait time If the answer is no, don t worry about it your goal isn t to try and eliminate all waits in the instance, because you can t do it.
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